RDMA/nldev: Export to user space number of contexts

[leonro@server ~]$ rdma res show
1: mlx5_0: pd 3 cq 5 qp 4 cm_id 0 mr 0 ctx 0

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>